Electroless plating refers to the autocatalytic or chemical reduction of metal ions in an aqueous solution to a metal which is deposited on a substrate. Typical electroless plating baths include electroless nickel and electroless copper, by way of example and not limitation. Components of the electroless plating bath include an aqueous solution of metal ions, reducing agents, complexing agents, bath stabilizers and a catalytic agent that operates at a specific metal ion concentration and within the specific temperature and pH range of the system. The base substrate, upon which the metal is plated, is usually catalytic in nature. Thus, the preferred preparation yields a substrate having a catalyzed surface and once the substrate is introduced into the electroless solution, uniform deposition begins. Minute amounts of the metal to be deposited on the substrate, i.e., nickel, further catalyze the reaction. After the original surfaces are coated with metal, the deposition is autocatalytic. Electroless deposition continues, provided that the metal ion and the reducing agent are replenished and the proper pH of the bath is maintained.
Electroless nickel plating generally deposits a nickel alloy onto a substrate which is capable of catalyzing the deposition of this alloy from a process solution containing nickel ions and a suitable chemical reducing agent which is capable of reducing nickel ions in solution to metallic nickel. These reducing agents typically include borohydride and hypophosphite ions. Typically, electroless nickel plating is carried out utilizing hypophosphite ions as the reducing agent. As the hypophosphite reduces the nickel at the catalytic surface, some phosphorus is co-deposited with the nickel yielding a nickel/phosphorus alloy containing between about 1 and 13% phosphorus. This alloy has unique properties in terms of corrosion resistance and (after heat treatment) hardness and wear resistance. Common applications of electroless nickel plating include electronics, computers, valves, aircraft parts, and copier and typewriter parts, by way of example and not limitation. In addition to the unique properties of nickel phosphorus alloys, using a chemical rather than an electrochemical method to produce these alloys has advantages in terms of deposit thickness distribution, giving a very uniform coating when compared to coatings produced by electrochemical methods.
In electroless plating, metal ions are reduced to metal by the action of chemical reducing agents. The reducing agents are oxidized in the process. The catalyst may be the substrate or metallic surface on the substrate, which allows the reduction-oxidation reactions to occur with the ultimate deposition of metal on the substrate.
The metal ion and reducer concentrations must be monitored and closely controlled in order to maintain proper ratios and to maintain the overall chemical balance within the plating bath. The electroless plating deposition rate is controlled by selecting the proper temperature, pH and metal ion/reducer concentrations. Complexing agents may be used as catalyst inhibitors to reduce the potential for spontaneous decomposition of the electroless bath.
The chemical reducing agent most commonly used in electroless plating is sodium hypophosphite, resulting in the generation of nickel phosphorus alloys. Others include sodium borohydride, dimethylamine borane, and N-diethylamine borane, which give nickel boron alloys and hydrazine and hydrogen, which give pure nickel alloys. Electroless nickel plating baths are generally of four types: (1) alkaline nickel phosphorus; (2) acid nickel phosphorus; (3) alkaline nickel boron; and (4) acid nickel boron. There are many potential and actual formulations for hypophosphite, borane and hydrazine reducing baths. However, in all cases the nickel ion is reduced to nickel metal and the reducing agent is mostly oxidized but, to a lesser extent, may also become part of the nickel deposit.
In spite of the many advantages of electroless nickel deposits from an engineering point of view, the deposition of electroless nickel generates significant waste. As the solution ages, it also becomes more viscous and so the plating speed and brightness of the deposit can be reduced. Most of the hypophosphite used to reduce the nickel becomes oxidized to orthophosphite which remains in the process solution and builds up in concentration until the bath must be replaced.
Nickel is maintained in the solution by the addition of a soluble nickel salt, which is typically nickel sulfate, nickel chloride, nickel acetate, nickel hypophosphite or combinations of one or more of the foregoing. The anion builds up and limits the life of the solution, along with the oxidation product from the reducing agent, which is typically orthophosphite. In a conventional system, this means that only about 60 g/L of nickel can be deposited before the concentration of salts reaches the solubility limits. In most commercial processes, the source of nickel is nickel sulfate so the process solution also builds up in sulfate ion. During operation of the bath, the pH tends to fall due to the generation of hydrogen atoms, which must be neutralized by the addition of an alkali such as ammonia, sodium hydroxide or potassium carbonate solutions. Again, these ions build up in concentration during bath operation. Eventually, the bath reaches saturation (or before this the rate of metal deposition becomes too slow for commercial operation) and has to be replaced.
One method of extending bath life is to add nickel to the bath as nickel hypophosphite rather than nickel sulfate. It can be manufactured by dissolution of nickel carbonate into hypophosphorous acid. However, nickel hypophosphite is a relatively expensive material and has limited solubility which gives rise to problems with bath maintenance.
In any electroless bath, an oxidation-reduction reaction occurs which results in oxidation products and metallic nickel. The pH decreases with removal of metal cations leaving anions of the nickel salt or complexing agent and the oxidation products of the reducing agents; i.e., hypophosphite to orthophosphite. The nickel ion and the reducing agent concentrations decrease with deposition. It is essential that the complexing agents, bath stabilizers and other additives remain in the bath at acceptable concentrations as the nickel is being deposited to prevent spontaneous decomposition of the bath and to minimize the number of chemicals that must be monitored and controlled.
Thus it can be seen that currently used electroless nickel baths have a limited life. The pH of the bath must be constantly adjusted with either an acid, usually sulfuric acid, or a base, usually ammonium hydroxide. The combination of hypophosphite oxidation producing orthophosphite and the reduction of nickel ions to metallic nickel usually results in excess acidity, which requires the addition of ammonium hydroxide to obtain the required pH.
The inventors of the present invention have discovered that by immersing a nickel anode either directly or indirectly using a selective ion membrane into the electroless nickel bath and passing an electric current through the bath, preferably using a divided cell arrangement with a perfluorinated cation exchange membrane to separate anolyte and catholyte, the nickel content of the plating bath can be maintained without the introduction of undesirable anions. This enables the bath to be used for more metal turnovers than a conventionally maintained bath which minimizes waste generation and improves consistency of plating rate.
Another unexpected benefit of using the process of the present invention for maintaining the nickel content of the electroless nickel bath is that the pH of the bath is far more stable. With a conventionally maintained electroless nickel bath, the pH of the bath falls during operation and additions of ammonia or potassium carbonate or hydroxide are required, which can sometimes generate localized instability of the bath. In the present invention, the bath is maintained by electrolytic dissolution of nickel and the pH remains relatively constant because the ionic balance of the solution is maintained by transport of hydrogen ions through the cation exchange membrane to the catholyte (to replace the hydrogen ions discharged at the cathode as hydrogen). This also contributes to increased bath life and stability.